Answer

Each number of the series is the sum of the squares of the individual digits contained in the preceding number. For example, the sum of the squares of the digits in the number 37 is (3 x 3) + (7 x 7) which is 58, the number in the series after 37. The answer is therefore 4, which is (2 x 2) + (0 x 0). This means that the series has started to repeat itself.